This invention relates generally to brushless dynamoelectric machines and, more particularly, to rotatable control wheel assemblies for such machines that carry components of the excitation control circuit.
Solid-state control circuit technology has been used for some time to provide brushless excitation for dynamoelectric machines such as synchronous motors. An example is disclosed in Hoffmann et al. U.S. Pat. No. 3,414,788, Dec. 3, 1968. By the use of such circuitry, large synchronous motors have been provided which do not require the use of brushes to excite the field winding of the motor. Such brushless machines have become quite popular since they offer advantages of reduced maintenance, the absence of brush replacement requirements, and the absence of sparks that could be hazardous in a combustible atmosphere. The system as it has been used previously has performed well, but is limited to motors of relatively large size, such as greater than about 5000 HP. It would be desirable to reduce the cost of the system in order to permit application to smaller size machines such as in the range from about 500 to 5000 HP.
The cost of the prior system is in large part due to the construction methods employed in the rotating control wheel which houses the electronic control circuitry and starting resistors. Reference is made to FIG. 1 which is a brushless motor control circuit in partial schematic form illustrating the exciter armature 10, diode rectifier 12, and other control circuitry 14, including starting resistor R, for supplying excitation to a motor field winding 16 as has been implemented in the past, such as in the above-referenced Hoffmann et al. Patent, which is incorporated herein by reference and shows further circuit details. The elements of FIG. 1 are all arranged for rotation together.
FIG. 2 shows how the control elements 12 and 14 of FIG. 1 have been previously arranged on a rotating structure. A wheel 17 is fabricated by rolling two aluminum rings 18 and 20 which are machined and welded to an aluminum plate 22 which forms a web connecting the outer ring 18 to the inner hub 20. The completed wheel is machined at the internal surface of the hub 20 which mates with the shaft 24 and on the outer and inner surfaces of the outer ring or rim 18, to the tolerances required for balance and shaft mounting. The view of FIG. 2 is of half of the structure which is substantially symmetrical about the center line of the shaft 24. The control wheel 17 provides support for the rotating control components which are fastened to it. Diode D is representative of the diodes and thyristors of elements 12 and 14 of FIG. 1. Each such component is mounted on a heat sink 26 secured to the rim 18 but insulated by insulation 28. Since the wheel is metallic, all diode heat sinks 28 and connecting cable and copper straps 30 must be insulated from the wheel 17. In addition, all bolted connections to the rim 18 which secure diode heat sinks 26 must be insulated.
Synchronous motors which are line-started (started in an induction mode) must have provision for current flow in the field winding during the starting sequence. For this purpose, systems such as that shown in FIG. 1 utilize a starting, or field discharge, resistor R which is connected across the field winding during the starting sequence when SCR1 is on as a result of a pulse at its gate electrode. FIG. 2 shows how the starting resistor is arranged in commercial apparatus presently being made. The current system uses commercially available magnesium-oxide insulated, cut-steel sheathed strip heaters 32 for these resistors. These heaters 32 are large elements which have little heat storage capacity and they also provide no hoop strength. This means that they heat quickly and have no excess mass to store heat and also require a large diameter control wheel since they cannot be rolled to a small diameter without damage and many heater elements are required to provide sufficient resistor capacity thus extending the length of the wheel.
The combination of bulky resistors, labor-intensive control wheel assembly (particularly for insulation of connections and bolts) and expensive, labor-intensive control wheel construction (particularly in required machinery or machining) have resulted in a brushless synchronous motor control system that is physically larger and more expensive than what would be preferred.
The inadequacies of prior systems like that of FIG. 2 have been recognized and attempts previously made to design a more cost effective unit. Heyne U.S. Pat. No. 3,845,369, Oct. 29, 1974, incorporated herein by reference, describes a system in which the strip heaters for the starting resistor are replaced with a wire resistor wound onto an aluminum control wheel. This configuration was practiced and was successful in reducing the size of the control wheel significantly. Problems developed, however, with the integrated resistor when it was discovered that the control circuitry in the system was not always operating properly. Corrective circuitry was provided to solve this problem as disclosed in Heyne et al. U.S. Pat. No. 4,038,589, July 26, 1977, incorporated herein by reference, but still resulted in an undesirable number of resistor failures due to inadequate cooling of the resistor. The various problems associated with the control circuitry of the brushless exciter have also given rise to a desire for a standardized control wheel on which there can be readily mounted a set of test collector rings to monitor the operation of the circuitry in either shop or field tests.
Among the features of the present invention, not all of which need to be used together to obtain advantages, is to provide a low-cost insulating, molded control wheel for mounting the circuit components and the starting resistor. The invention decreases control wheel fabrication and assembly costs and further, with the use of a new starting resistor arrangement, reduces the size of the control wheel. Materials are available, such as glass polyesters, which are structurally strong and can be molded to close tolerances without significant shrinkage. The type of materials referred to are generally those that have been previously used for molded, insulating cases for electrical distribution and interruption equipment and in such applications as high-pressure arc chutes for circuit breakers. These materials have proven to be inexpensive to manufacture in a variety of shapes and are extremely rugged so they are comparable to metals such as aluminum. The ability to mold these materials to close tolerances allows the use of intricately shaped pieces which cost little to manufacture and require no manual labor or machining after molding.
In one form of implementing the invention, the control wheel comprises a mass of molded insulating material that provides, in a unitary structure, a central hub for mounting with the shaft, a radial plate and an axially running rim spaced from the hub. The electrical components of the excitation control circuitry are mounted directly on the inner surface of the rim and require no additional insulation. The starting resistor is a wire-wound element wound on the outer surface of the rim and held thereon by molded insulating clamp elements and/or glass banding for radial support. Additionally, the outer surface of the rim is channeled and the coils of the resistor are spaced in a manner to provide vent passages therethrough.
In alternate forms of the invention, the unitary mass of molded insulating material comprises the control wheel in combination with at least one metal support element which is electrically isolated by the molded insulating material from the elements of the rectifier bridge and starting control circuit.
The resistor arrangement above described is of such benefit as to be advantageous to use even with presently configured metal control wheel where adequate insulation is provided between the resistor and the surface of the wheel.
Numerous additional features and advantages of the present invention will be understood from the ensuing description. While the description is primarily directed to brushless systems for synchronous motors, it will be understood that some features, such as the molded insulating control wheel, are applicable to brushless systems for a dynamoelectric machine which can be either a synchronous motor, synchronous generator, or DC motor. In instances such as large synchronous generators where other means for starting is provided, the invention may be practiced without the use of a starting resistor as implemented in the embodiments described herein.